Liquid toner dispersion is a stabilised dispersion for use in a printing process. It differs from other ink dispersions such as dispersions for offset printing and inkjet compositions, on the basis of the particles it contains. Whereas an inkjet composition generally comprises pigment as such, the liquid toner dispersion comprises marking particles comprising pigment that is mixed with or is embedded in a polyester resin binder. The resulting particles suitably have a diameter in the range of 0.5-5.0 μm whereas the pigment particle size in inkjet and offset is below 500 nm.
In such liquid toner printing process a digital printing apparatus as described in US patent application no 2011/0249990 can be used. The liquid toner dispersion is typically transferred from feeding means, such as a feed roller to a first member via a second member to a substrate. Those members are for instance cylindrical rollers, but may have another form. The first member is also known as a development roller. The second member is also referred to as a photoconductor. Further intermediate members may be present, for instance between the photoconductor and the substrate.
One of the complexities of the liquid toner process is the stability of the liquid toner dispersion. In relation thereto, a dispersant is used. The use of hyper-dispersants seems beneficial. These hyper-dispersants comprise an anchor group and a stabilising group. The anchor group is anchored on the polymer particle surface by single-point or—typically—multipoint anchoring. Both acrylates and amines are known as anchor group. The stabilising group grafted onto the anchor group extends in non-aqueous system to provide steric stability. One specific example of a known hyper-dispersant is a graft copolymer with a polyethylene-imine (PEI) as the anchor group and poly(12-hydroxy stearic acid) as the solvent group in aliphatic hydrocarbon continuous phases. This graft copolymer of a hydroxylated fatty acid is commercially available, for instance from Lubrizol under the tradename Solsperse™. Other known hyper-dispersants are available from Tianlong Chemicals under the tradename of Tilosperse™.
The use of Solsperse™ dispersing agents are for instance disclosed in US2007/0258731A1. This application relates to the preparation of a liquid toner dispersion. Herein, toner material is milled in a fatty acid monoester, and therefore homogeneously distributed therein (par [0104]). The milled toner material is thereafter dispersed in the carrier liquid, which is an unsaturated fatty acid triglyceride (par [0038]). A dispersant is added, which is preferably a polymer dispersant, with a weight-average molecular weight of 1000 to 100,000, more preferably 5000 to 80,000 (par [0076]). The polymer dispersant is more preferably a polyamine aliphatic polycondensate, which has a weight-average molecular weight of 5000-80,000 (par [0324]). Specific examples are Solsperse™ 13940 and Solsperse™ 11200.
It has been found by the inventors of the present invention in the course of investigations in relation to liquid toner dispersions, that the stability requirements are manifold. The dispersion should evidently be stable during and after preparation, i.e. during storage and upon application to the first member. But the liquid toner process further requires that the fusing is not hindered or disturbed by the presence of the same dispersant. Moreover, the dispersion should not be sensitive to an artefact called caking, after charging the dispersion and/or discharging the dispersion and when the toner layer is mechanically stressed (e.g. when blade cleaning is performed).
In order to transfer the liquid toner dispersion from the first member to the second member, the liquid toner dispersion is typically charged. Due to this charging process, the transfer may be selective, such that merely a desired image is transferred from the first member to the second member. Charging and compaction of the liquid toner dispersion however also has an impact on the stability of the dispersion. As a consequence hereof, an issue occurs that is known as ‘caking’. Particularly, polymer ‘marking’ particles in the dispersion tend to form lumps in the dispersion resulting in a liquid with a non-uniform distribution of marking particles. This caking often results in an increase of the viscosity of the liquid dispersion. This viscosity increase is significant, and could be a tenfold increase or even more, resulting in a more difficult liquid system to be transported. Liquid developer dispersion that shows caking cannot be used for printing as such and needs to be treated first in order to re-obtain a homogeneously dispersed liquid toner which has similar physical properties like conductivity and viscosity as the starting liquid developer dispersion. It is thought that caking is the result of marking particles that come so close into each other's neighbourhood on the developing member, so that they start to feel each other's presence and start interacting with each other. Caking can also be the result of injecting charge and applying high shearing forces which are typically present when a thin layer of liquid developer dispersion passes through a very narrow gap between two (rotating) members of the printing apparatus or huge (microsized) mechanical interaction like a cleaning blade scraping on a circular surface.
It is therefore an object of the invention to obtain a liquid toner dispersion that would meet all needs. The hyperdispersant should in particular be capable of stabilising the initial liquid toner dispersion, but it should not disturb the fusing process and it should not give rise to significant caking, at least less than the known Solsperse™ dispersants, such as Solsperse™ 11000 and Solsperse™ 13940.